In general, a method of manufacturing a high-strength press-hardened product includes heating a steel material of which hardenability is improved by adding B, Mo, and Cr etc. at a high temperature of about 900° C. above an Ac3 transformation point to be completely changed into an austenite state, hot-forming the steel sheet at one time into a product shape with a press die, and rapidly cooling it into a martensite structure.
As well known in the related art, a steel sheet is easy to form because its ductility is increased when being heated at a high temperature. Thus, the machinability of a steel sheet manufactured by press hardening is slightly better than that of typical steel sheets for machining and considerably better than that of high-strength steel.
Further, a steel sheet manufactured by press hardening has very high strength (above 1,400 MPa) such that it is significantly advantageous in terms of specific strength, obtained by dividing yield strength by density, and thus can considerably contribute to reducing weight of vehicles. Further, the steel sheet manufactured by press hardening is used to manufacture ultra high-strength parts that are difficult to form, because there is little spring back after machining.
It is required to heat a steel sheet at about 900° C. or more for several minutes to transform the steel sheet to an austenite state in press hardening process, and this should be automated for an efficient process.
As shown in FIG. 1, a blank B is heated in a heating furnace for press hardening process of the related art for several minutes. The blank B to be hardened is obtained from a wound steel sheet coil C and, heated through a straight furnace O for achieving an automated process, carried by a robot R to a press P, and then pressed therein.
However, the straight furnace system includes a several tens of meters straight unit to maintain a predetermined temperature for a predetermined time so as to achieve complete austenite transformation. Thus, efficiency for heating to desired temperature is low, and a large factory area is required to install the equipment.
Further, in the existing straight furnace system, since the steel sheet blank is conveyed on a conveying unit such as a roller table, a temperature difference occurs between the upper portion and the portion contacting the conveying unit. Thus, the quality of a product made of such steel sheet is deteriorated.